The method of producing cores that became known by the name of “Cold Box method” or “Ashland method” has attained great significance in the foundry industry. To this end, two-component polyurethane systems are employed for binding a refractory mold base material. The polyol component is composed of a polyol comprising at least two OH groups per molecule, the isocyanate component is composed of a diisocyanate/polyisocyanate comprising at least two NCO groups per molecule. The binder is cured with the aid of basic catalysts. Liquid bases can be admixed to the binder prior to molding so as to cause the two components to react with each other (U.S. Pat. No. 3,676,392). Another option is that of conducting gaseous tertiary amines through the mold material/binder mixture after shaping (U.S. Pat. No. 3,409,579).
According to U.S. Pat. No. 3,676,392 and U.S. Pat. No. 3,409,579, phenolic resins are used as polyols, which are obtained by condensing phenols with aldehydes, preferably formaldehyde, in the liquid phase at temperatures up to approximately 130° C. in the presence of catalytic concentrations of metal ions.
U.S. Pat. No. 3,485,797 describes the production of such phenolic resins in detail. In addition to unsubstituted phenol, it is possible to use substituted phenols, preferably o-cresol and p-nonyl phenol (see, for example, U.S. Pat. No. 4,590,229). According to EP 0177871 A2, aliphatic monoalcohol groups containing phenolic resins modified with one to eight carbon atoms can be used as another reaction component. Alkoxylation is intended to give the binders increased thermal stability.
The solvents that are used for the polyol component are primarily mixtures of high-boiling polar solvents (such as esters and ketones) and high-boiling aromatic hydrocarbons. In contrast, the isocyanates are preferably dissolved or placed in high-boiling aromatic hydrocarbons.
U.S. Pat. No. 6,883,587 B2 describes binders that contain monomeric carbodiimides in the isocyanate component. These are used to improve moisture resistance. The structure of the monomeric carbodiimides is R1—N═C═N═R2, where R1 and R2 each denote hydrogen, alkyl, alkenyl, cycloalkyl, aryl, naphthyl, alkyl-substituted aryl, or aralkyl.
There is great interest in lowering emissions during the production of the core, core storage and casting in the molds. This is not only in the interest of protecting the environment and in the interest of occupational safety, but also so as to increase machine availability, for example during permanent mold casting, due to reduced condensate formation and therefore extended cleaning intervals. So as to reduce emission loads during production of the core or mold, drying of the core or mold, and casting in molds and cores, it is desirable to reduce the amount of material that can be pyrolized, which is to say the binder that is applied, while nonetheless preserving the required properties.
In terms of strength, attention must be paid in particular to sufficient initial strengths, in particular if integral molds are to be assembled into complex core packages immediately after production using (semi)automatic systems or are to be inserted into metallic permanent molds.
It was therefore the object of the invention to provide a mold material mixture, by way of which shaped bodies for the casting industry can be produced, which have lower emissions (condensates) as compared to shaped bodies that were produced using a mold material mixture that is provided with a conventional binder.